1. Field of the Invention
The present invention relates to the field of integrated circuit design, and more particularly to integrated circuits having on-chip, one or more oscillators and/or resistance dependent circuits such as MOSFET resistance dependent circuits.
2. Prior Art
In a Sample Data System, there is always the need to band limit the incoming analog signal and sample it periodically. Band-limiting of the analog signal is performed through a suitable lowpass or a bandpass filter, and the periodic sampling is performed by sampling circuitry with an accurate periodic time-base. The periodic time base is generated from a suitable oscillator.
One form of monolithic implementation of a suitable filter is the continuous time active MOSFET-RC type. In an MOSFET-RC filter, MOSFET transistors are used in the non-saturation region with a control voltage on the gate to generate an effective resistance. Processing tolerances of integrated MOSFET transistor and capacitor parameters may create +/-50% variation in the filter time constants. To maintain tight tolerances on the filter time constants, it becomes necessary to have a control loop, referenced to a stable reference, to control the time constants of the filter over process and ambient changes. In implementations where the time-base oscillator is also specified to be implemented in a monolithic form with the filter, the same process tolerances will create +/-50% variations on the oscillator frequency. To achieve an accurate and stable oscillation frequency, the oscillator also needs to be referenced to a stable and accurate reference. The present invention provides a simple and unique way of achieving these ends in a monolithic device.
In prior art situations where it is necessary to have a trimmed current-to-voltage transfer function (T=V/I), most implementations develop a voltage Vo across a trimming resistor Rt by passing a current I through the resistor. By trimming the resistor Rt, an accurate voltage Vo is developed or an accurate current to voltage transfer function is achieved. FIG. 1 shows a general implementation scheme. In integrated circuit implementations, the trimming resistor Rt may be formed as a series or parallel combination of multiple resistors with switches, as shown in FIG. 2a and FIG. 2b, respectively, or a single resistor may be used where trimming is performed through laser cutting as shown in FIG. 2c. The switches may be active or passive. Active switches may either be bipolar or MOS transistors or zener diodes. Passive switches may each just be a thin interconnect line, such as metal.
When MOS transistors are used as switches, a fair amount of silicon area is used up because large MOS switches need to be made to have their turn-on resistance small compared to the trim resistor, as otherwise, trimming is not accurate. Bipolar switches have an inherent turn-on voltage Vsat, whereby they are non-ideal switches and hence create inaccurate trimming. Zener zapping and laser trimming are clumsy and expensive because of the sophisticated external hardware that is required. Both zener zapping and laser trimming have long term reliability problems, and are not reprogrammable.